1. Field of the Invention
The present invention relates to a universal joint, for example, which is used in a propeller shaft and a drive shaft of a vehicle, and which is provided with a boot disposed between an inner and an outer element of the universal joint, sealing and holding lubricant such grease or the like therewithin and preventing dust, water or the like from entering from the outside.
2. Description of the Related Art
Regarding a conventional universal joint provided with a boot, there is, for example, a structure described in Japanese Patent Application Laid-Open (JP-A) No. 9-14284. The universal joint `a` described in this publication is a tripod type uniform joint which is used in a drive shaft of a vehicle, and is provided with a cylindrical outer `b` and an inner `c` fitted into the outer `b`, as shown in FIG. 12.
The inner `c` is provided with a drive shaft `d`, a ring-like member which is mounted to an end portion of the drive shaft `d` and three rollers `e` which are rotatably supported to three support shafts extending from the ring-like member outward in a diametrical direction and arranged in a peripheral direction at a uniform interval, respectively. Further, three guide grooves which are formed for respectively receiving three rollers `e` of the inner `c` along an axial direction are provided in an inner periphery of the outer `b`. Then, a torque is transmitted between the inner `c` and the outer `b` via three rollers `e` which are engaged with the guide grooves.
On the contrary, a boot `f` mounted between the inner `c` and the outer `b` of the universal joint `a` is constituted by a cylindrical boot metal fitting `g` and a flexible elastic body `h` fixed to one end portion of the boot metal fitting `g`. The boot metal fitting `g` is fitted to an outer periphery of the outer `b` and the elastic body `h` is fitted to an outer periphery of the drive shaft `d`. An O-ring `i` is attached to a groove formed in the fitting portion of the outer `b`. In the manner mentioned above, the inside of the joint is kept airtight by the boot `f` to prevent the lubricant charged therewithin from leaking out and dust, water or the like from entering thereinto.
In this case, in the prior art mentioned above, when inserting the inner `c` to which the boot `f` is fitted into the inside of the outer `b` so as to assemble the inner `c` and the outer `b`, the boot metal fitting `g` is fitted to the outer periphery of the outer `b`. At this time, since the air within the boot `f` is not taken out while the inner periphery of the boot metal fitting `g` is brought into contact with the O-ring `i`, the portion between the boot metal fitting `g` and the outer `b` is sealed by the O-ring `i`, and when the boot metal fitting `g` is further inserted to reach the final fitting position, the air is compressed and the pressure thereof is increased. As a result, the elastic body `h` is expanded in a manner shown by a double dot chain line in FIG. 12 due to the increased air pressure.
In the case that the elastic body `h` is used in this state, the elastic body `h` and the drive shaft `d` may rub against each other when the inner `c` and the outer `b` relatively move in an axial direction, and an angle formed by an axis of the outer `b` and an axis of the inner `c` changes, so that durability of the boot `f` is deteriorated. Further, since the elastic body `h` is expanded when the sealed air is expanded due to a centrifugal force of the grease during rotation of the universal joint `a` and a temperature increase, the elastic body `h` and the inner `c` rub against each other under a stronger force.
Then, in order to avoid the matter mentioned above, an air pressure adjusting operation may be performed for taking out the air sealed and pressurized within the boot `f` after the fitting operation between the boot metal fitting `g` and the outer `b` is completed, and the assembly between the inner `c` and the outer `b` is completed. This air pressure adjusting operation is performed, for example, by inserting a tool into the fitting portion between the elastic body `h` and the drive shaft `d`, thereby forming an interval for communicating the inside of the boot `f` with the open air between the both so as to discharge the air within the boot to the open air.
However, since the adjusting operation not only increases the process and labor of assembling the universal joint but also damages the boot `f` during the adjusting operation, the adjusting operation must be performed very carefully. Because of the care required, there is generated a reduction of efficiency of the operation of assembling the universal joint `a`.